Electron-Rich N-Heterocyclic Silylene (NHSi)-Iron Complexes: Synthesis, Structures, and Catalytic Ability of an Isolable Hydridosilylene-Iron Complex

The first electron-rich N-heterocyclic silylene (NHSi)-iron(0) complexes are reported. The synthesis of the starting complex is accomplished by reaction of the electron-rich Fe-0 precursor [(dmpe)(2)Fe(PMe3)] 1 (dmpe = 1,2-bis(dimethylphosphino)ethane) with the N-heterocyclic chlorosilylene LSiCl (L = PhC((NBu)-Bu-t)(2)) 2 to give, via Me3P elimination, the corresponding iron complex [(dmpe)(2)Fe(<-:Si(Cl)L)] 3. Reaction of in situ generated 3 with MeLi afforded [(dmpe)(2)Fe(<-:Si(Me)L)] 4 under salt metathesis reaction, while its reaction with Li[BHEt3] yielded [(dmpe)(2)Fe(<-:Si(H)L)] 5, a rare example of an isolable Si-II hydride complex and the first such example for iron. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analyses. DFT calculations further characterizing the bonding situation between the Si-II and Fe-0 centers were also carried out, whereby multiple bonding character is detected in all cases (Wiberg Bond Index > 1). For the first time, the catalytic activity of a Si-II hydride complex was investigated. Complex 5 was used as a precatalyst for the hydrosilylation of a variety of ketones in the presence of (EtO)(3)SiH as a hydridosilane source. In most cases excellent conversions to the corresponding alcohols were obtained after workup. The reaction pathway presumably involves a ketone-assisted 1,2-hydride transfer from the Si-II to Fe-0 center, as a key elementary step, resulting in a betaine-like silyliumylidene intermediate. The appearance of the latter intermediate is supported by DFT calculations, and a mechanistic proposal for the catalytic process is presented.